Electrochemical production of peroxides



Patented Aug. 24, 1937 UNITED STATES PATENT OFFICE ELECTROCHEMICAL PRODUCTION OF PEROXIDES Virginia No Drawing. Application May 11, 1935, Serial No. 20,991

4 Claims.

This invention relates to improvements in the electrolytic production of peroxides. More particularly, the invention relates to improvements in the recovery of hydrogen peroxide, as magnesium peroxide, from alkaline cell liquors containing hydrogen peroxide. The invention includes improvements in the regeneration of electrolytes used in the electrolytic production of peroxides. Although of general application in the electrolytic 10 production of peroxides, the improvements of this invention are particularly useful in connection with diaphragm cells.

This invention is of special value and application in connection with the electrolytic processes described in my prior applications Serial No. 566,155 filed September 30, 1931 (upon which United States Patent 2,000,815 was granted on May 7, 1935) and Serial No. 751,031, filed November 1, 1934. The processes described in said applications involve the production of hydrogen peroxide by a reaction in an electrolytic cell between oxygen absorbed by a cathode containing activated carbon and cathodic hydrogen produced at the cathode by electrolysis of a soluble metal compound. A diphragm cell may be employed in the practice of these processes.

Magnesium peroxide has some special advantages, as applied for pharmaceutical and bleaching purposes for example, in that in the presence of water it becomes hydrogen peroxide and magnesium hydroxide and the latter, because of its limited solubility, exerts only a very limited alkaline action.

In carrying out, in diaphragm cells, the processes of my said prior applications, the catholyte potassium chloride and using high current density to secure a good yield of concentrated hydrogen peroxide, the catholyte, containing the hydrogen peroxide formed, will also reach a normality of from '7 to 9 with respect to potassium hydroxide. Evolution of chlorine at the anode coupled with this migration of potassium ions impoverishes the anolyte with respect to potassium chloride.

The hydrogen peroxide content of alkaline cell liquors so produced is with advantage recovered,

in accordance with this invention, by reacting the liquor, catholyte withdrawn from a diaphragm cell for example, with an appropriate compound of magnesium to precipitate magnesium peroxide.

The oxide, the hydroxide and the salts of magnesium are useful in this connection. The reaction may be effected within the cell, but it is more advantageously efiected externally of the cell. The reaction is with advantage efl'ected at a moderate or a low temperature, from about 40 C. down to 0 C., or even lower temperatures down to the freezing point of the solution. When magnesium oxide, magnesium hydroxide or mixtures of the two are used, the reaction proceeds with direct precipitation of the magnesium peroxide. When a magnesium salt or salts is used, the reaction also involves conversion of the hydroxide present in the cell liquor to the corresponding salt. The precipitated magnesium peroxide may be separated from the solution, washed if necessary, and dried in any convenient manner.

A magnesium compound having the same anion as the compound subjected to electrolysis is with advantage used to precipitate magnesium peroxide from the alkaline cell liquor. For example, if potassium chloride is the compound subjected to electrolysis, then a magnesium chloride is used, or with potassium sulfate, magnesium sulfate is used, or with potassium hydroxide, magnesium hydroxide, or oxide, is used.

When a magnesium compound having the same anion as the compound subjected to electrolysis I is used to precipitate magnesium peroxide from the alkaline cell liquor, the liquor is regenerated and becomes available for return to the electrolysis. Operation in this manner does not impair the balance of the fundamental electrolysis, but on the contrary even assists, in providing a peculiarly appropriate way to overcome the impoverishment of the anolyte previously mentioned in connection with diaphragm cell operation, in maintaining once established conditions of operation.

When an electrolyte so regenerated is to be returned to the electrolysis, appropriate steps should be taken to avoid or to minimize the presence of magnesium compounds in the returned electrolyte. If magnesium compounds are present in any substantial amount in the returned electrolyte, in diaphragm operation, the migration of magnesium ions into the catholyte tends to form a precipitate consisting of a mixture of magnesium peroxide and magnesium oxide which, precipitating on the cathode in a peculiarly insoluble form, increases the potential drop across the cell and almost inevitably involves clogging up of the diaphragm.

The manner in which the regenerated solution is freed from magnesium compound before being returned to the electrolysis is not important. For example, the amount of the magnesium compound used to effect the reaction can be carefully limited to avoid any excess and the solubility of the magnesium peroxide formed can be reduced to a minimum by the addition, for example, of alcohol or ammonia or triethanol amine to the reacted solution. Such precipitants may be separated from the solution before return to the electrolysis in any convenient manner. For example, alcohol or ammonia can be distilled from the solution or such precipitants can be extracted, for example, with benzol or ether.

When a magnesium compound including water of crystallization is used to effect the precipitation of magnesium peroxide from the alkaline cell liquor and the regenerated solution is to be returned to the electrolysis, such distillation to separate an added precipitant such as alcohol or ammonia may with advantage be carried further to include the distillation of sufficient Water from the solution to bring it to a concentration corresponding to that of the electrolyte supplied to the electrolysis, to that of the anolyte in diaphragm cell operation for example. Any contaminating hydroxide included with the precipitated and separated magnesium peroxide is with advantage converted to the corresponding salt by the addition of the appropriate amount of one or more salts of weak bases to lower the alkalinity of the magnesium peroxide. Ammonium salts, pyridine salts and salts of triethanol amine, for example, are useful in this connection. Soluble salts formed as the result of such additions to the precipitated and separated magnesium peroxide may be removed from the product, for example, by washing.

The following examples will further illustrate the invention:

Example I The electrolysis is carried out as described in my said prior application Serial Number 566,155, using as the catholyte a normal potassium chloride solution and as the anolyte a potassium chloride solution saturated at the temperature at which the electrolysis is conducted. A large volume of anolyte is used to facilitate the operation and a small volume of catholyte is maintained in the cathode compartment to assist in recovery of hydrogen peroxide in high concentration and to minimize decomposition of hydrogen peroxide once formed. By maintaining the catholyte at a low temperature while using current densities of from 30 to 50 amperes or more per square decimeter of cathode area, a high yield of hydrogen peroxide is obtained and, at the same time, the catholyte soon reaches a normality with respect to potassium hydroxide of from '7 to 9, the volume of the catholyte increasing because of the migration of water and potassium ions. Chlorine is evolved at the anode. A portion of the catholyte, for example a portion corresponding to the increase in volume resulting from the migration just mentioned, is withdrawn from the cathode compartment from time to time and reacted with magnesium chloride at a moderate temperature. Either anhydrous magnesium chloride or magnesium chloride including water of crystallization may be used, but if the anhydrous salt is used it is important that temperature increase, and particularly local temperature increases, be avoided. The addition of anhydrous magnesium chloride in a very finely divided form and strong cooling of the solution undergoing reaction assist in this connection. Magnesium peroxide, which may be contaminated with magnesium hydroxide, is precipitated. The addition of magnesium chl0- ride in excess of that reacting to precipitate magnesium peroxide should be avoided. Either during the reaction or after the reaction, a precipitant reducing the solubility of magnesium peroxide may be' added. The precipitated magnesium peroxide is separated from the solution, with a minimum of delay, washed if necessary, with alcohol for example, and dried at a moderate temperature. After separation of precipitated magnesium peroxide, the solution is freed from any added precipitant and, if necessary, concentrated until approximately saturated with respect to potassium chloride. The solution is then returned to the anode compartment. If concentration has been carried to a point such that potassium chloride begins to separate, any such separated potassium chloride may be returned to the anode compartment with the returned solution. The impoverished anolyte is thus periodically restored to its original condition of saturation with respect to potassium chloride.

Example II The electrolysisis carried out as described in my said prior application Serial Number 566,155, us ing as the catholyte an alkali metal hydroxide, potassium hydroxide for example. Provision being made for vigorous agitation of the catholyte in the cell in order to retard precipitation of magnesium peroxide on the cathode, magnesium oxide or magnesium hydroxide may be added to the catholyte in the cell. More advantageously, however, a portion of the catholyte is withdrawn from the cell from time to time, reacted with magnesium oxide or magnesium hydroxide while maintaining a low temperature, as low as or somewhat lower than 0 C. for example. The presence of the alkali metal hydroxide minimizes any tendency toward solution of magnesium compounds in the reaction liquor. In order to suppress decomposition of precipitatedmagnesium peroxide because of the alkalinity of the solution, the precipitated magnesium peroxide should -be separated from the solution with a minimum of delay and promptly washed free from alkali metal hydroxide, with alcohol for example. Any tendency toward decomposition because of the presence of contaminating alkali metal hydroxide can also be overcome by adding to the separated magnesium peroxide an appropriate amount of one or more salts of weak bases. Any unreacted magnesium oxide or magnesium hydroxide may also thus be converted to soluble salts removable from the product, for example by washing.

I claim:

1. In a process involving the production in an electrolytic cell of hydrogen peroxide by a reaction between oxygen absorbed by a cathode containing activated carbon and hydrogen produced at said cathode by the electrolysis of an electrolyte containing a soluble metal salt, the improvement which comprises employing a current density of at least 30 amperes per square decimeter of cathode area, withdrawing electrolyte from the cell in the neighborhood of the cathode, reacting the electrolyte thus withdrawn with a magnesium compound having the same anion as the soluble metal salt to form magnesium peroxide, separating the magnesium peroxide thus formed from ode with a catholyte and an anolyte respectively in contact therewith by reaction between oxygen absorbed at the cathode and hydrogen produced in the catholyte whereby the hydrogen peroxide 10 content of the catholyte becomes enriched, the

improvement which comprises employing a current density of at least 30 amperes per square decimeter of cathode area, withdrawing the enriched catholyte from the cell, reacting the 15 catholyte thus withdrawn with a magnesium compound having the same anion as the anolyte to form a precipitate of magnesium peroxide, removing-magnesium peroxide thus formed from the withdrawn catholyte, and returning the re- 20 sulting purified catholyte to the cell as anolyte.

3. In a process involving the production in an electrolytic cell of hydrogen peroxide by reaction between oxygen absorbed by a cathode and cathodic hydrogen in a solution in contact with the cathode and containing an alkali metal compound whereby the hydrogen peroxide content of the solution is increased, the improvement which comprises employing a current density of at least 30 amperes per square foot of cathode area, removing solution having a hydrogen ion concentration corresponding to a potassium hydroxide normality of from 7-9 from the cell in the neighborhood of the cathode, reacting the solution thus withdrawn with a magnesium compound corresponding to the anion of the compound in the solution to form a precipitate of magnesium peroxide, separating the precipitate from the solution and returning the resulting purified solution to the electrolytic cell.

4. Process according to claim 3 in which the alkali metal compound in the solution is a chloride and magnesium chloride is used to precipitate hydrogen peroxide from the solution withdrawn from the cell.

ERNST BERL. 

